hg-review is distributed under the same license as Mercurial itself: GPL version 2 or any later version.
If you want to create a program that works with hg-review you should look at Mercurial’s License FAQ page to learn about how this might affect you.
The basic idea is:
Note that the last item (using hg-review’s internal Python API) is probably the one you won’t want to do anyway, since the Python API is not stable.
If you have any questions please email Steve, but remember that he’s not a lawyer and might not have a fast answer for tricky questions.